1. Field of the Invention
The invention relates generally to a current-in-the-plane spin-valve magnetoresistive sensor, and more particularly to such a sensor with an improved cap over the sensor free layer.
2. Description of the Related Art
One type of conventional magnetoresistive sensor, often called a “spin-valve” (SV) sensor, has a stack of layers that include two ferromagnetic layers separated by a nonmagnetic electrically-conductive spacer layer. One ferromagnetic layer has its magnetization direction fixed, typically by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance. This is because the scattering of the electrons shared by the free and pinned layers is dependent on the orientation of their electron spin relative to the magnetization direction of the layer they penetrate.
The SV magnetoresistive sensor used in all current magnetic recording hard disk drives operates with the sense current directed parallel to the planes of the layers in the sensor layer stack, so it is referred to as a current-in-the-plane (CIP) sensor. In a disk drive CIP-SV read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance.
CIP-SV read heads are formed by successive deposition of the layers on a substrate, typically a layer of electrically insulating material that serves as the first read gap layer. When the fixed ferromagnetic layer is deposited before the free ferromagnetic layer, the head is called a “bottom-pinned” head. In a bottom-pinned CIP-SV head with the free layer on top, a cap is formed on top of the free layer before deposition of the second gap layer.
The conventional cap over the top free layer was typically a single capping layer of metal, such as Ta. More recently, nonmagnetic metal-oxides, e.g. TaOx or AlOx, have been used instead of conventional metals to cap the free layer in bottom-pinned CIP-SV read heads. Sensors with these metal-oxide single capping layers show a higher ΔR (Rmax−Rmin, where Rmax and Rmin are the sensor resistances with the magnetizations antiparallel and parallel, respectively) and a higher magnetoresisitance (ΔR/R). The nonmagnetic metal-oxide capping layers are sometimes called “specular reflection” layers because they act to confine electrons and thus increase the occurrence of spin-dependent scattering of electrons at the interface of the spacer layer and the free layer.
When a metal-oxide capping layer is used, it is often necessary to have a thin buffer layer of metal between the free layer and the capping layer to prevent oxidation of the free layer and thus degradation of the magnetoresistance of the sensor. However, because the metal buffer layer is electrically conducting, some of the sense current is shunted away from the free and pinned layers, thereby reducing the sensor magnetoresistance.
What is needed is a bottom-pinned CIP-SV magnetoresistive sensor with an improved free layer cap.